The Collaborative International Dictionary of English v.0.48:

Nullification \Nul`li*fi*ca"tion\, n. [L. nullificatio contempt.
   See Nullify.]
   The act of nullifying; a rendering void and of no effect, or
   of no legal effect.
   [1913 Webster]

   Right of nullification (U. S. Hist.), the right claimed in
      behalf of a State to nullify or make void, by its
      sovereign act or decree, an enactment of the general
      government which it deems unconstitutional.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nullification
    n 1: the states'-rights doctrine that a state can refuse to
         recognize or to enforce a federal law passed by the United
         States Congress
    2: the act of nullifying; making null and void; counteracting or
       overriding the effect or force of something [syn:
       nullification, override]